This excerpt taken from the XEL 10-K filed Feb 27, 2009.
Sec. 4.2 Special Provisions for PSCo and SPS SERP Participation. For Participants who participated in the PSCo SERP or the SPS SERP on the day before the Effective Date, the Normal Retirement Benefit under Sec. 4.1 shall not be less than the Actuarial Equivalent (expressed in the form payable under Sec. 4.1) the accrued benefit determined under the PSCo SERP or the SPS SERP (whichever covered the Participant on the Effective Date), determined by assuming that the Participant separated from service on May 1, 2000. However, whether the Participant is vested in such benefit shall be determined pursuant to Sec. 4.3 of this Plan as of the date the Participants actual separation from service occurs.